Vikings, Vikings, Vikings! “eastern” ancestry in the whole Baltic Iron Age

vikings-middle-age

Open access Population genomics of the Viking world, by Margaryan et al. bioRxiv (2019), with a huge new sampling from the Viking Age.

Interesting excerpts (emphasis mine, modified for clarity):

To understand the genetic structure and influence of the Viking expansion, we sequenced the genomes of 442 ancient humans from across Europe and Greenland ranging from the Bronze Age (c. 2400 BC) to the early Modern period (c. 1600 CE), with particular emphasis on the Viking Age. We find that the period preceding the Viking Age was accompanied by foreign gene flow into Scandinavia from the south and east: spreading from Denmark and eastern Sweden to the rest of Scandinavia. Despite the close linguistic similarities of modern Scandinavian languages, we observe genetic structure within Scandinavia, suggesting that regional population differences were already present 1,000 years ago.

Maps illustrating the following texts have been made based on data from this and other papers:

  • Maps showing ancestry include only data from this preprint (which also includes some samples from Sigtuna).
  • Maps showing haplogroup density include Vikings from other publications, such as those from Sigtuna in Krzewinska et al. (2018), and from Iceland in Ebenesersdóttir et al. (2018).
  • Maps showing haplogroups of ancient DNA samples based on their age include data from all published papers, but with slightly modified locations to avoid overcrowding (randomized distance approx. ± 0.1 long. and lat.).

middle-ages-europe-y-dna
Y-DNA haplogroups in Europe during the Viking expansions (full map). See other maps from the Middle Ages.

We find that the transition from the BA to the IA is accompanied by a reduction in Neolithic farmer ancestry, with a corresponding increase in both Steppe-like ancestry and hunter-gatherer ancestry. While most groups show a slight recovery of farmer ancestry during the VA, there is considerable variation in ancestry across Scandinavia. In particular, we observe a wide range of ancestry compositions among individuals from Sweden, with some groups in southern Sweden showing some of the highest farmer ancestry proportions (40% or more in individuals from Malmö, Kärda or Öland).

Ancestry proportions in Norway and Denmark on the other hand appear more uniform. Finally we detect an influx of low levels of “eastern” ancestry starting in the early VA, mostly constrained among groups from eastern and central Sweden as well as some Norwegian groups. Testing of putative source groups for this “eastern” ancestry revealed differing patterns among the Viking Age target groups, with contributions of either East Asian- or Caucasus-related ancestry.

saami-ancestry-vikings
Ancestry proportions of four-way models including additional putative source groups for target groups for which three-way fit was rejected (p ≤ 0.01);

Overall, our findings suggest that the genetic makeup of VA Scandinavia derives from mixtures of three earlier sources: Mesolithic hunter-gatherers, Neolithic farmers, and Bronze Age pastoralists. Intriguingly, our results also indicate ongoing gene flow from the south and east into Iron Age Scandinavia. Thus, these observations are consistent with archaeological claims of wide-ranging demographic turmoil in the aftermath of the Roman Empire with consequences for the Scandinavian populations during the late Iron Age.

Genetic structure within Viking-Age Scandinavia

We find that VA Scandinavians on average cluster into three groups according to their geographic origin, shifted towards their respective present-day counterparts in Denmark, Sweden and Norway. Closer inspection of the distributions for the different groups reveals additional complexity in their genetic structure.

vikings-danish-ancestry
Natural neighbor interpolation of “Danish ancestry” among Vikings.

We find that the ‘Norwegian’ cluster includes Norwegian IA individuals, who are distinct from both Swedish and Danish IA individuals which cluster together with the majority of central and eastern Swedish VA individuals. Many individuals from southwestern Sweden (e.g. Skara) cluster with Danish present-day individuals from the eastern islands (Funen, Zealand), skewing towards the ‘Swedish’ cluster with respect to early and more western Danish VA individuals (Jutland).

Some individuals have strong affinity with Eastern Europeans, particularly those from the island of Gotland in eastern Sweden. The latter likely reflects individuals with Baltic ancestry, as clustering with Baltic BA individuals is evident in the IBS-UMAP analysis and through f4-statistics.

vikings-norwegian-ancestry
Natural neighbor interpolation of “Norwegian ancestry” among Vikings.

For more on this influx of “eastern” ancestry see my previous posts (including Viking samples from Sigtuna) on Genetic and linguistic continuity in the East Baltic, and on the Pre-Proto-Germanic homeland based on hydrotoponymy.

Baltic ancestry in Gotland

Genetic clustering using IBS-UMAP suggested genetic affinities of some Viking Age individuals with Bronze Age individuals from the Baltic. To further test these, we quantified excess allele sharing of Viking Age individuals with Baltic BA compared to early Viking Age individuals from Salme using f4 statistics. We find that many individuals from the island of Gotland share a significant excess of alleles with Baltic BA, consistent with other evidence of this site being a trading post with contacts across the Baltic Sea.

vikings-finnish-ancestry
Natural neighbor interpolation of “Finnish ancestry” among Vikings.

The earliest N1a-VL29 sample available comes from Iron Age Gotland (VK579) ca. AD 200-400 (see Iron Age Y-DNA maps), which also proves its presence in the western Baltic before the Viking expansion. The distribution of N1a-VL29 and R1a-Z280 (compared to R1a in general) among Vikings also supports a likely expansion of both lineages in succeeding waves from the east with Akozino warrior-traders, at the same time as they expanded into the Gulf of Finland.

vikings-y-dna-haplogroup-r1a-z280-over-r1a
Density of haplogroup R1a-Z280 (samples in pink) overlaid over other R1a samples (in green, with R1a-Z284 in cyan) among Vikings.

Vikings in Estonia

(…) only one Viking raiding or diplomatic expedition has left direct archaeological traces, at Salme in Estonia, where 41 Swedish Vikings who died violently were buried in two boats accompanied by high-status weaponry. Importantly, the Salme boat-burial predates the first textually documented raid (in Lindisfarne in 793) by nearly half a century. Comparing the genomes of 34 individuals from the Salme burial using kinship analyses, we find that these elite warriors included four brothers buried side by side and a 3rd degree relative of one of the four brothers. In addition, members of the Salme group had very similar ancestry profiles, in comparison to the profiles of other Viking burials. This suggests that this raid was conducted by genetically homogeneous people of high status, including close kin. Isotope analyses indicate that the crew descended from the Mälaren area in Eastern Sweden thus confirming that the Baltic-Mid-Swedish interaction took place early in the VA.

vikings-swedish-ancestry
Natural neighbor interpolation of “Swedish ancestry” among Vikings.

Viking samples from Estonia show thus ancient Swedes from the Mälaren area, which proves once again that hg. N1a-VL29 (especially subclade N1a-L550) and tiny proportions of so-called “Siberian ancestry” expanded during the Early Iron Age into the whole Baltic Sea area, not only into Estonia, and evidently not spreading with Balto-Finnic languages (since the language influence is in the opposite direction, east-west, Germanic > Finno-Samic, during the Bronze Age).

N1a-VL29 lineages spread again later eastwards with Varangians, from Sweden into north-eastern Europe, most likely including the ancestors of the Rurikid dynasty. Unsurprisingly, the arrival of Vikings with Swedish ancestry into the East Baltic and their dispersal through the forest zone didn’t cause a language shift of Balto-Finnic, Mordvinic, or East Slavic speakers to Old Norse, either…

NOTE. For N1a-Y4339 – N1a-L550 subclade of Swedish origin – as main haplogroup of modern descendants of Rurikid princes, see Volkov & Seslavin (2019) – full text in comments below. Data from ancient samples show varied paternal lineages even among early rulers traditionally linked to Rurik’s line, which explains some of the discrepancies found among modern descendants:

  • A sample from Chernihiv (VK542) potentially belonging to Gleb Svyatoslavich, the 11th century prince of Tmutarakan/Novgorod, belongs to hg. I2a-Y3120 (a subclade of early Slavic I2a-CTS10228) and has 71% “Modern Polish” ancestry (see below).
  • Izyaslav Ingvarevych, the 13th century prince of Dorogobuzh, Principality of Volhynia/Galicia, is probably behind a sample from Lutsk (VK541), and belongs to hg. R1a-L1029 (a subclade of R1a-M458), showing ca. 95% of “Modern Polish” ancestry.
  • Yaroslav Osmomysl, the 12th century Prince of Halych (now in Western Ukraine), was probably of hg. E1b-V13, yet another clearly early Slavic haplogroup.

vikings-y-dna-haplogroup-n1a
Density of haplogroup N1a-VL29, N1a-L550 (samples in pink, most not visible) among Vikings. Samples of hg. R1b in blue, hg. R1a in green, hg. I in orange.

Finnish ancestry

Firstly, modern Finnish individuals are not like ancient Finnish individuals, modern individuals have ancestry of a population not in the reference; most likely Steppe/Russian ancestry, as Chinese are in the reference and do not share this direction. Ancient Swedes and Norwegians are more extreme than modern individuals in PC2 and 4. Ancient UK individuals were more extreme than Modern UK individuals in PC3 and 4. Ancient Danish individuals look rather similar to modern individuals from all over Scandinavia. By using a supervised ancient panel, we have removed recent drift from the signal, which would have affected modern Scandinavians and Finnish populations especially. This is in general a desirable feature but it is important to check that it has not affected inference.

ancient-modern-finns-steppe
PCA of the ancient and modern samples using the ancient palette, showing different PCs. Modern individuals are grey and the K=7 ancient panel surrogate populations are shown in strong colors, whilst the remaining M-K=7 ancient populations are shown in faded colors.

The story for Modern-vs-ancient Finnish ancestry is consistent, with ancient Finns looking much less extreme than the moderns. Conversely, ancient Norwegians look like less-drifted modern Norwegians; the Danish admixture seen through the use of ancient DNA is hard to detect because of the extreme drift within Norway that has occurred since the admixture event. PC4 vs PC5 is the most important plot for the ancient DNA story: Sweden and the UK (along with Poland, Italy and to an extent also Norway) are visibly extremes of a distribution the same “genes-mirror-geography” that was seen in the Ancient-palette analysis. PC1 vs PC2 tells the same story – and stronger, since this is a high variance-explained PC – for the UK, Poland and Italy.

Uniform manifold approximation and projection (UMAP) analysis of the VA and other ancient samples.

Evidence for Pictish Genomes

The four ancient genomes of Orkney individuals with little Scandinavian ancestry may be the first ones of Pictish people published to date. Yet a similar (>80% “UK ancestry) individual was found in Ireland (VK545) and five in Scandinavia, implying that Pictish populations were integrated into Scandinavian culture by the Viking Age.

Our interpretation for the Orkney samples can be summarised as follows. Firstly, they represent “native British” ancestry, rather than an unusual type of Scandinavian ancestry. Secondly, that this “British” ancestry was found in Britain before the Anglo-Saxon migrations. Finally, that in Orkney, these individuals would have descended from Pictish populations.

vikings-british-ancestry
Natural neighbor interpolation of “British ancestry” among Vikings.

(…) ‘UK’ represents a group from which modern British and Irish people all receive an ancestry component. This information together implies that within the sampling frame of our data, they are proxying the ‘Briton’ component in UK ancestry; that is, a pre-Roman genetic component present across the UK. Given they were found in Orkney, this makes it very likely that they were descended from a Pictish population.

Modern genetic variation within the UK sees variation between ‘native Briton’ populations Wales, Scotland, Cornwall and Ireland as large compared to that within the more ‘Anglo-Saxon’ English. This is despite subsequent gene flow into those populations from English-like populations. We have not attempted to disentangle modern genetic drift from historically distinct populations. Roman-era period people in England, Wales, Ireland and Scotland may not have been genetically close to these Orkney individuals, but our results show that they have a shared genetic component as they represent the same direction of variation.

Density of haplogroup R1b-L21 (samples in red), overlaid over all samples of hg. R1b among Vikings (R1b-U106 in green, other R1b-L151 in deep red). To these samples one may add the one from Janakkala in south-western Finland (AD ca. 1300), of hg. R1b-L21, possibly related to these population movements.

For more on Gaelic ancestry and lineages likely representing slaves among early Icelanders, see Ebenesersdóttir et al. (2018).

Y-DNA

As in the case of mitochondrial DNA, the overall distribution profile of the Y chromosomal haplogroups in the Viking Age samples was similar to that of the modern North European populations. The most frequently encountered male lineages were the haplogroups I1, R1b and R1a.

Haplogroup I (I1, I2)

The distribution of I1 in southern Scandinavia, including a sample from Sealand (VK532) ca. AD 100 (see Iron Age Y-DNA maps) proves that it had become integrated into the West Germanic population already before their expansions, something that we already suspected thanks to the sampling of Germanic tribes.

vikings-y-dna-haplogroup-i
Density of haplogroup I (samples in orange) among Vikings. Samples of hg. R1b in blue, hg. R1a in green, N1a in pink.
vikings-y-dna-haplogroup-i1-over-i
Density of haplogroup I1 (samples in red) overlaid over all samples of hg. I among Vikings.

Haplogroup R1b (M269, U106, P312)

Especially interesting is the finding of R1b-L151 widely distributed in the historical Nordic Bronze Age region, which is in line with the estimated TMRCA for R1b-P312 subclades found in Scandinavia, despite the known bottleneck among Germanic peoples under U106. Particularly telling in this regard is the finding of rare haplogroups R1b-DF19, R1b-L238, or R1b-S1194. All of that points to the impact of Bell Beaker-derived peoples during the Dagger period, when Pre-Proto-Germanic expanded into Scandinavia.

Also interesting is the finding of hg. R1b-P297 in Troms, Norway (VK531) ca. 2400 BC. R1b-P297 subclades might have expanded to the north through Finland with post-Swiderian Mesolithic groups (read more about Scandinavian hunter-gatherers), and the ancestry of this sample points to that origin.

However, it is also known that ancestry might change within a few generations of admixture, and that the transformation brought about by Bell Beakers with the Dagger Period probably reached Troms, so this could also be a R1b-M269 subclade. In fact, the few available data from this sample show that it comes from the natural harbour Skarsvågen at the NW end of the island Senja, and that its archaeologist thought it was from the Viking period or slightly earlier, based on the grave form. From Prescott (2017):

In 1995, Prescott and Walderhaug tentatively argued that a dramatic transformation took place in Norway around the Late Neolithic (2350 BCE), and that the swift nature of this transition was tied to the initial Indo-Europeanization of southern and coastal Norway, at least to Trøndelag and perhaps as far north as Troms. (…)

The Bell Beaker/early Late Neolithic, however, represents a source and beginning of these institution and practices, exhibits continuity to the following metal age periods and integrated most of Northern Europe’s Nordic region into a set of interaction fields. This happened around 2400 BCE, at the MNB to LN transition.

NOTE. This particular sample is not included in the maps of Viking haplogroups.

vikings-y-dna-haplogroup-r1b
Density of haplogroup R1b (samples in blue) among Vikings. Samples of hg. I in orange, hg. R1a in green, N1a in pink.
vikings-y-dna-haplogroup-r1b-U106-over-r1b
Density of haplogroup R1b-U106 (samples in green) overlaid over all samples of hg. R1b (other R1b-L23 samples in red) among Vikings.
vikings-y-dna-haplogroup-r1b-P312-over-r1b
Density of R1b-L151 (xR1b-U106) (samples in deep red) overlaid over all samples of hg. R1b (R1b-U106 in green, other R1b-M269 in blue) among Vikings.

Haplogroup R1a (M417, Z284)

The distribution of hg. R1a-M417, in combination with data on West Germanic peoples, shows that it was mostly limited to Scandinavia, similar to the distribution of I1. In fact, taking into account the distribution of R1a-Z284 in particular, it seems even more isolated, which is compatible with the limited impact of Corded Ware in Denmark or the Northern European Plain, and the likely origin of R1a-Z284 in the expansion with Battle Axe from the Gulf of Finland. The distribution of R1a-Z280 (see map above) is particularly telling, with a distribution around the Baltic Sea mostly coincident with that of N1a.

vikings-y-dna-haplogroup-r1a
Density of haplogroup R1a (samples in green) among Vikings. Samples of hg. R1b in blue, of hg. I in orange, N1a in pink.
vikings-y-dna-haplogroup-r1a-z284-over-r1a
Density of haplogroup R1a-Z284 (samples in cyan) overlaid over all samples of hg. R1a (in green, with R1a-Z280 in pink) among Vikings.

Other haplogroups

Among the ancient samples, two individuals were derived haplogroups were identified as E1b1b1-M35.1, which are frequently encountered in modern southern Europe, Middle East and North Africa. Interestingly, the individuals carrying these haplogroups had much less Scandinavian ancestry compared to the most samples inferred from haplotype based analysis. A similar pattern was also observed for less frequent haplogroups in our ancient dataset, such as G (n=3), J (n=3) and T (n=2), indicating a possible non-Scandinavian male genetic component in the Viking Age Northern Europe. Interestingly, individuals carrying these haplogroups were from the later Viking Age (10th century and younger), which might indicate some male gene influx into the Viking population during the Viking period.

vikings-italian-ancestry
Natural neighbor interpolation of “Italian ancestry” among Vikings.

As the paper says, the small sample size of rare haplogroups cannot distinguish if these differences are statistically relevant. Nevertheless, both E1b samples have substantial Modern Polish-like ancestry: one sample from Gotland (VK474), of hg. E1b-L791, has ca. 99% “Polish” ancestry, while the other one from Denmark (VK362), of hg. E1b-V13, has ca. 35% “Polish”, ca. 35% “Italian”, as well as some “Danish” (14%) and minor “British” and “Finnish” ancestry.

Given the E1b-V13 samples of likely Central-East European origin among Lombards, Visigoths, and especially among Early Slavs, and the distribution of “Polish” ancestry among Viking samples, VK362 is probably a close description of the typical ancestry of early Slavs. The peak of Modern Polish-like ancestry around the Upper Pripyat during the (late) Viking Age suggests that Poles (like East Slavs) have probably mixed since the 10th century with more eastern peoples close to north-eastern Europeans, derived from ancient Finno-Ugrians:

vikings-polish-ancestry
Natural neighbor interpolation of “Polish ancestry” among Vikings.

Similarly, the finding of R1a-M458 among Vikings in Funen, Denmark (VK139), in Lutsk, Poland (VK541), and in Kurevanikha, Russia (VK160), apart from the early Slav from Usedom, may attest to the origin of the spread of this haplogroup in the western Baltic after the Bell Beaker expansion, once integrated in both Germanic and probably Balto-Slavic populations, as well as intermediate Bronze Age peoples that were eventually absorbed by their expansions. This contradicts, again, my simplistic initial assessment of R1a-M458 expansion as linked exclusively (or even mainly) to Balto-Slavs.

antiquity-europe-y-dna
Y-DNA haplogroups in Europe during Antiquity (full map). See other maps of cultures and ancient DNA from Antiquity.

Related

Iberia: East Bell Beakers spread Indo-European languages; Celts expanded later

iberia-migrations-celts

New paper (behind paywall), The genomic history of the Iberian Peninsula over the past 8000 years, by Olalde et al. Science (2019).

NOTE. Access to article from Reich Lab: main paper and supplementary materials.

Abstract:

We assembled genome-wide data from 271 ancient Iberians, of whom 176 are from the largely unsampled period after 2000 BCE, thereby providing a high-resolution time transect of the Iberian Peninsula. We document high genetic substructure between northwestern and southeastern hunter-gatherers before the spread of farming. We reveal sporadic contacts between Iberia and North Africa by ~2500 BCE and, by ~2000 BCE, the replacement of 40% of Iberia’s ancestry and nearly 100% of its Y-chromosomes by people with Steppe ancestry. We show that, in the Iron Age, Steppe ancestry had spread not only into Indo-European–speaking regions but also into non-Indo-European–speaking ones, and we reveal that present-day Basques are best described as a typical Iron Age population without the admixture events that later affected the rest of Iberia. Additionally, we document how, beginning at least in the Roman period, the ancestry of the peninsula was transformed by gene flow from North Africa and the eastern Mediterranean.

Interesting excerpts:

From the Bronze Age (~2200–900 BCE), we increase the available dataset (6, 7, 17) from 7 to 60 individuals and show how ancestry from the Pontic-Caspian steppe (Steppe ancestry) appeared throughout Iberia in this period (Fig. 1, C and D), albeit with less impact in the south (table S13). The earliest evidence is in 14 individuals dated to ~2500–2000 BCE who coexisted with local people without Steppe ancestry (Fig. 2B). These groups lived in close proximity and admixed to form the Bronze Age population after 2000 BCE with ~40% ancestry from incoming groups (Fig. 2B and fig. S6).

Y-chromosome turnover was even more pronounced (Fig. 2B), as the lineages common in Copper Age Iberia (I2, G2, and H) were almost completely replaced by one lineage, R1b-M269. These patterns point to a higher contribution of incoming males than females, also supported by a lower proportion of nonlocal ancestry on the X-chromosome (table S14 and fig. S7), a paradigm that can be exemplified by a Bronze Age tomb from Castillejo del Bonete containing a male with Steppe ancestry and a female with ancestry similar to Copper Age Iberians.

iberian-adna

For the Iron Age, we document a consistent trend of increased ancestry related to Northern and Central European populations with respect to the preceding Bronze Age (Figs. 1, C and D, and 2B). The increase was 10 to 19% (95% confidence intervals given here and in the percentages that follow) in 15 individuals along the Mediterranean coast where non-Indo-European Iberian languages were spoken; 11 to 31% in two individuals at the Tartessian site of La Angorrilla in the southwest with uncertain language attribution; and 28 to 43% in three individuals at La Hoya in the north where Indo-European Celtiberian languages were likely spoken (fig. S6 and tables S11 and S12).

This trend documents gene flow into Iberia during the Late Bronze Age or Early Iron Age, possibly associated with the introduction of the Urnfield tradition (18). Unlike in Central or Northern Europe, where Steppe ancestry likely marked the introduction of Indo-European languages (12), our results indicate that, in Iberia, increases in Steppe ancestry were not always accompanied by switches to Indo-European languages.

I think it is obvious they are extrapolating the traditional (not that well-known) linguistic picture of Iberia during the Iron Age, believing in continuity of that picture (especially non-Indo-European languages) during the Urnfield period and earlier.

What this data shows is, as expected, the arrival of Celtic languages in Iberia after Bell Beakers and, by extension, in the rest of western Europe. Somewhat surprisingly, this may have happened during the Urnfield period, and not during the La Tène period.

Also important are the precise subclades:

We thus detect three Bronze Age males who belonged to DF27 (154, 155), confirming its presence in Bronze Age Iberia. The other Iberian Bronze Age males could belong to DF27 as well, but the extremely low recovery rate of this SNP in our dataset prevented us to study its true distribution. All the Iberian Bronze Age males with overlapping sequences at R1b-L21 were negative for this mutation. Therefore, we can rule out Britain as a plausible proximate origin since contemporaneous British males are derived for the L21 subtype.


New open access paper Survival of Late Pleistocene Hunter-Gatherer Ancestry in the Iberian Peninsula, by Villalba-Mouco et al. Cell (2019):

BAL0051 could be assigned to haplogroup I1, while BAL003 carries the C1a1a haplogroup. To the limits of our typing resolution, EN/MN individuals CHA001, CHA003, ELT002 and ELT006 share haplogroup I2a1b, which was also reported for Loschbour [73] and Motala HG [13], and other LN and Chalcolithic individuals from Iberia [7, 9], as well as Neolithic Scotland, France, England [9], and Lithuania [14]. Both C1 and I1/ I2 are considered typical European HG lineages prior to the arrival of farming. Interestingly, CHA002 was assigned to haplogroup R1b-M343, which together with an EN individual from Cova de Els Trocs (R1b1a) confirms the presence of R1b in Western Europe prior to the expansion of steppe pastoralists that established a related male lineage in Bronze Age Europe [3, 6, 9, 13, 19]. The geographical vicinity and contemporaneity of these two sites led us to run genomic kinship analysis in order to rule out any first or second degree of relatedness. Early Neolithic individual FUC003 carries the Y haplogroup G2a2a1, commonly found in other EN males from Neolithic Anatolia [13], Starçevo, LBK Hungary [18], Impressa from Croatia and Serbia Neolithic [19] and Czech Neolithic [9], but also in MN Croatia [19] and Chalcolithic Iberia [9].

See also

Reproductive success among ancient Icelanders stratified by ancestry

iceland-pca

New paper (behind paywall), Ancient genomes from Iceland reveal the making of a human population, by Ebenesersdóttir et al. Science (2018) 360(6392):1028-1032.

Abstract and relevant excerpts (emphasis mine):

Opportunities to directly study the founding of a human population and its subsequent evolutionary history are rare. Using genome sequence data from 27 ancient Icelanders, we demonstrate that they are a combination of Norse, Gaelic, and admixed individuals. We further show that these ancient Icelanders are markedly more similar to their source populations in Scandinavia and the British-Irish Isles than to contemporary Icelanders, who have been shaped by 1100 years of extensive genetic drift. Finally, we report evidence of unequal contributions from the ancient founders to the contemporary Icelandic gene pool. These results provide detailed insights into the making of a human population that has proven extraordinarily useful for the discovery of genotype-phenotype associations.

icelanders
Shared drift of ancient and contemporary Icelanders. (A) Scatterplot of D-statistics reflecting Iceland-specific drift. To aid interpretation, we included values for ancient British-Irish Islanders and a subset of contemporary individuals (who were correspondingly removed from the reference populations).

We estimated the mean Norse ancestry of the settlement population (24 pre-Christians and one early Christian) as 0.566 [95% confidence interval (CI) 0.431–0.702], with a nonsignificant difference betweenmales (0.579) and females (0.521). Applying the same ADMIXTURE analysis to each of the 916 contemporary Icelanders, we obtained a mean Norse ancestry of 0.704 (95% CI 0.699–0.709). Although not statistically significant (t test p = 0.058), this difference is suggestive. A similar difference ofNorse ancestry was observed with a frequency-based weighted least-squares admixture estimator (16), 0.625 [Mean squared error (MSE) = 0.083] versus 0.74 (MSE = 0.0037). Finally, the D-statistic test D(YRI, X; Gaelic, Norse) also revealed a greater affinity between Norse and contemporary Icelanders (0.0004, 95% CI 0.00008–0.00072) than between Norse and ancient Icelanders (−0.0002, 95% CI −0.00056–0.00015). This observation raises the possibility that reproductive success among the earliest Icelanders was stratified by ancestry, as genetic drift alone is unlikely to systematically alter ancestry at thousands of independent loci (fig. S10). We note that many settlers of Gaelic ancestry came to Iceland as slaves, whose survival and freedom to reproduce is likely to have been constrained (17). Some shift in ancestry must also be due to later immigration from Denmark, which maintained colonial control over Iceland from 1380 to 1944 (for example, in 1930 there were 745 Danes out of a total population of 108,629 in Iceland) (18).

icelander-admixture
Shared drift of ancient and contemporary Icelanders. (B) Estimated Norse,
Gaelic, and Icelandic ancestry for ancient Icelanders using ADMIXTURE
in supervised mode.

Five pre-Christian Icelanders (VDP-A5, DAVA9, NNM-A1, SVK-A1 and TGS-A1) fall just outside the space occupied by contemporary Norse in Fig. 3A. That these individuals show a stronger signal of drift shared with contemporary Icelanders is also apparent in the results of ADMIXTURE, run in supervised mode with three contemporary reference populations (Norse, Gaelic, and Icelandic) (Fig. 3B). The correlation between the proportion of Icelandic ancestry from this analysis and PC1 in Fig. 2A is |r| = 0.913.(…)

(…) as the five ancient Icelanders fall well within the cluster of contemporary Scandinavians (Fig. 3C), we conclude that they, or close relatives, likely contributed more to the contemporary Icelandic gene pool than the other pre-Christians. We note that this observation is consistent with the inference that settlers of Norse ancestry had greater reproductive success than those of Gaelic ancestry.

icelanders-y-dna
Haplogroup data, from the paper. Image modified by me, with those close to Gaelic and British/Irish samples (see above Scatterplot of D-statistics and ADMIXTURE data) marked in fluorescent: yellow closer to Gaelic, green less close.

Ancient Icelanders show a clear relation with the typically Norse Y-DNA distribution: I1 / R1a-Z284 / R1b-U106.

  • Among R1a, the picture is uniformly of R1a-Z284 (at least five of the seven reported).
  • There are six samples of I1, with great variation in subclades.
  • Among R1b-L51 subclades (ten samples), there are U106 (at least one sample), L21 (three samples), and another P312 (L238); see above the relationship with those clustering closely with Gaelic samples, marked in fluorescent, which is compatible with Gaelic settlers (predominantly of R1b-L21 lineages) coming to Iceland as slaves.

Probably not much of a surprise, coming from Norse speakers, but they are another relevant reference for comparison with samples of East Germanic tribes, when they appear.

Also, the first reported Klinefelter (XXY) in ancient DNA (sample ID is YGS-B2).

Related:

Yleaf: software for human Y-chromosomal haplogroup inference from next generation sequencing data

portugal-bronze-age-admixture

Brief communication (behind paywall) Yleaf: software for human Y-chromosomal haplogroup inference from next generation sequencing data, by Arwin Ralf, Diego Montiel González, Kaiyin Zhong, and Manfred Kayser, Mol Biol Evol (2018), msy032.

Abstract

Next generation sequencing (NGS) technologies offer immense possibilities given the large genomic data they simultaneously deliver. The human Y chromosome serves as good example how NGS benefits various applications in evolution, anthropology, genealogy and forensics. Prior to NGS, the Y-chromosome phylogenetic tree consisted of a few hundred branches, based on NGS data it now contains many thousands. The complexity of both, Y tree and NGS data provide challenges for haplogroup assignment. For effective analysis and interpretation of Y-chromosome NGS data, we present Yleaf, a publically available, automated, user-friendly software for high-resolution Y-chromosome haplogroup inference independently of library and sequencing methods.

Here is a link to the software Yleaf’s website, from the Department of Genetic Identification, at the University of Erasmus Medical Center.

yleaf-martiniano
Summary of NGS datasets used for automated NRY haplogrouping with Yleaf

Excerpt:

In the time of NGS (or massively parallel sequencing, MPS), the amount of genomic data produced and made publically available is rapidly expanding, providing valuable resources for many areas of research and applications. Due to its haploid nature and male-specific inheritance, the non-recombining part of the human Y-chromosome (NRY) is highly suitable for phylogenetic studies and for addressing questions in evolution, anthropology, population history, genealogy and forensics (Jobling & Tyler-Smith, 2017). Over recent years, NGS data allowed the phylogenetic NRY tree to dramatically increase in size and complexity (Hallast et al. 2014; Poznik et al. 2016). The two most comprehensive tree versions ISOGG (http://www.isogg.org/tree) and Yfull (https://www.yfull.com/tree) currently contain thousands of branches. However, the complexity of both, Y tree and NGS data provide immense challenges for NRY haplogroup assignment, which reflects a key element in many NRY applications. Here we introduce Yleaf, a Phyton-based, easy-to-use, publically-available software tool for effective NRY single nucleotide polymorphism (SNP) calling and subsequent NRY haplogroup inference from NGS data. By comparative whole genome data analysis, we demonstrate high concordance of Yleaf in NRY-SNP calling compared to well-established tools such as SAMtools/BCFtools (Li et al. 2009), and GATK (McKenna, et al. 2010) as well as improved performance of Yleaf in NRY haplogroup assignment relative to previously developed tools such as clean_tree (Ralf et al. 2015), AMY-tree (Van Geystelen et al. 2015), and yHaplo (Poznik, 2016).

Yleaf allows analyzing NRY sequence data from many types of NGS libraries i.e., whole genomes, whole exomes, large genomic regions, and large numbers of targeted amplicons. Several modifications relative to our previously developed clean_tree tool (Ralf et al. 2015) were implemented to optimize the performance especially relevant for extremely large NGS datasets such as whole genomes. For instance, Yleaf extracts the Y-chromosomal reads prior to further processing and uses multi-threading, a batch option is included too. Importantly, Yleaf provides drastically increased haplogroup resolution i.e., from Downloaded from 530 positions defining 432 NRY haplogroups with clean_tree (Ralf et al. 2015) to over 41,000 positions defining 5353 haplogroups with Yleaf. For a detailed method description see the supplementary material.

Featured image: From Martiniano et al. (2017).

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Migrations painted by Irish and Scottish genetic clusters, and their relationship with British and European ones

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Interesting and related publications, now appearing in pairs…

1. The Irish DNA Atlas: Revealing Fine-Scale Population Structure and History within Ireland, by Gilbert et al., in Scientific Reports (2017).

Abstract:

The extent of population structure within Ireland is largely unknown, as is the impact of historical migrations. Here we illustrate fine-scale genetic structure across Ireland that follows geographic boundaries and present evidence of admixture events into Ireland. Utilising the ‘Irish DNA Atlas’, a cohort (n = 194) of Irish individuals with four generations of ancestry linked to specific regions in Ireland, in combination with 2,039 individuals from the Peoples of the British Isles dataset, we show that the Irish population can be divided in 10 distinct geographically stratified genetic clusters; seven of ‘Gaelic’ Irish ancestry, and three of shared Irish-British ancestry. In addition we observe a major genetic barrier to the north of Ireland in Ulster. Using a reference of 6,760 European individuals and two ancient Irish genomes, we demonstrate high levels of North-West French-like and West Norwegian-like ancestry within Ireland. We show that that our ‘Gaelic’ Irish clusters present homogenous levels of ancient Irish ancestries. We additionally detect admixture events that provide evidence of Norse-Viking gene flow into Ireland, and reflect the Ulster Plantations. Our work informs both on Irish history, as well as the study of Mendelian and complex disease genetics involving populations of Irish ancestry.

european-ancestry-british-isles
The European ancestry profiles of 30 Irish and British clusters. (a) The total ancestry contribution summarised by majority European country of origin to each of the 30 Irish and British clusters. (b) (left) The ancestry contributions of 19 European clusters that donate at least 2.5% ancestry to any one Irish or British cluster. (right) The geographic distribution of the 19 European clusters, shown as the proportion of individuals in each European region belonging to each of the 19 European clusters. The proportion of individuals form each European region not a member of the 19 European clusters is shown in grey. Total numbers of individuals from each region are shown in white text. Not all Europeans included in the analysis were phenotyped geographically. The figure was generated in the statistical software language R46, version 3.4.1, using various packages. The map of Europe was sourced from the R software package “mapdata” (https://CRAN.R-project.org/package=mapdata).

2. New preprint on BioRxiv, Insular Celtic population structure and genomic footprints of migration, by Byrne, Martiniano et al. (2017).

Abstract:

Previous studies of the genetic landscape of Ireland have suggested homogeneity, with population substructure undetectable using single-marker methods. Here we have harnessed the haplotype-based method fineSTRUCTURE in an Irish genome-wide SNP dataset, identifying 23 discrete genetic clusters which segregate with geographical provenance. Cluster diversity is pronounced in the west of Ireland but reduced in the east where older structure has been eroded by historical migrations. Accordingly, when populations from the neighbouring island of Britain are included, a west-east cline of Celtic-British ancestry is revealed along with a particularly striking correlation between haplotypes and geography across both islands. A strong relationship is revealed between subsets of Northern Irish and Scottish populations, where discordant genetic and geographic affinities reflect major migrations in recent centuries. Additionally, Irish genetic proximity of all Scottish samples likely reflects older strata of communication across the narrowest inter-island crossing. Using GLOBETROTTER we detected Irish admixture signals from Britain and Europe and estimated dates for events consistent with the historical migrations of the Norse-Vikings, the Anglo-Normans and the British Plantations. The influence of the former is greater than previously estimated from Y chromosome haplotypes. In all, we paint a new picture of the genetic landscape of Ireland, revealing structure which should be considered in the design of studies examining rare genetic variation and its association with traits.

Here are some interesting excerpts (emphasis mine):

Population structure in Ireland

The geographical distribution of this deep subdivision of Leinster resembles pre-Norman territorial boundaries which divided Ireland into fifths (cúige), with north Leinster a kingdom of its own known as Meath (Mide) [15]. However interpreted, the firm implication of the observed clustering is that despite its previously reported homogeneity, the modern Irish population exhibits genetic structure that is subtly but detectably affected by ancestral population structure conferred by geographical distance and, possibly, ancestral social structure.

ChromoPainter PC1 demonstrated high diversity amongst clusters from the west coast, which may be attributed to longstanding residual ancient (possibly Celtic) structure in regions largely unaffected by historical migration. Alternatively, genetic clusters may also have diverged as a consequence of differential influence from outside populations. This diversity between western genetic clusters cannot be explained in terms of geographic distance alone.

In contrast to the west of Ireland, eastern individuals exhibited relative homogeneity; (…) The overall pattern of western diversity and eastern homogeneity in Ireland may be explained by increased gene flow and migration into and across the east coast of Ireland from geographically proximal regions, the closest of which is the neighbouring island of Britain.

Analysis of variance of the British admixture component in cluster groups showed a significant difference (p < 2×10-16), indicating a role for British Anglo-Saxon admixture in distinguishing clusters, and ChromoPainter PC2 was correlated with the British component (p < 2×10-16), explaining approximately 43% of the variance. PC2 therefore captures an east to west Anglo-Celtic cline in Irish ancestry. This may explain the relative eastern homogeneity observed in Ireland, which could be a result of the greater English influence in Leinster and the Pale during the period of British rule in Ireland following the Norman invasion, or simply geographic proximity of the Irish east coast to Britain. Notably, the Ulster cluster group harboured an exceptionally large proportion of the British component (Fig 1D and 1E), undoubtedly reflecting the strong influence of the Ulster Plantations in the 17th century and its residual effect on the ethnically British population that has remained.

ireland-population-structure
Fine-grained population structure in Ireland. (A) fineSTRUCTURE clustering dendrogram for 1,035 Irish individuals. Twenty-three clusters are defined, which are combined into cluster groups for clusters that are neighbouring in the dendrogram, overlapping in principal component space (B) and sampled from regions that are geographically contiguous. Details for each cluster in the dendrogram are provided in S1 Fig. (B) Principal components analysis (PCA) of haplotypic similarity, based on ChromoPainter coancestry matrix for Irish individuals. Points are coloured according to cluster groups defined in (A); the median location of each cluster group is plotted. (C) Map of Irelandshowing the sampling location for a subset of 588 individuals analysed in (A) and (B), coloured by cluster group. Points have been randomly jittered within a radius of 5 km to preserve anonymity. Precise sampling location for 44 Northern Irish individuals from the People of the British Isles dataset was unknown; these individuals are plotted geometrically in a circle. (D) “British admixture component” (ADMIXTURE estimates; k=2) for Irish cluster groups. This component has the largest contribution in ancient Anglo-Saxons and the SEE cluster. (E) Linear regression of principal component 2 (B) versus British admixture component (r2 = 0.43; p < 2×10-16). Points are coloured by cluster group. (Standard error for ADMIXTURE point estimates presented in S11 Fig.)

On the genetic structure of the British Isles

The genetic substructure observed in Ireland is consistent with long term geographic diversification of Celtic populations and the continuity shown between modern and Early Bronze Age Irish people

Clusters representing Celtic populations harbouring less Anglo-Saxon influence separate out above and below SEE on PC4. Notably, northern Irish clusters (NLU), Scottish (NISC, SSC and NSC), Cumbria (CUM) and North Wales (NWA) all separate out at a mutually similar level, representing northern Celtic populations. The southern Celtic populations Cornwall (COR), south Wales (SWA) and south Munster (SMN) also separate out on similar levels, indicating some shared haplotypic variation between geographically proximate Celtic populations across both Islands. It is notable that after the split of the ancestrally divergent Orkney, successive ChromoPainter PCs describe diversity in British populations where “Anglo-saxonization” was repelled [22]. PC3 is dominated by Welsh variation, while PC4 in turn splits North and South Wales significantly, placing south Wales adjacent to Cornwall and north Wales at the other extreme with Cumbria, all enclaves where Brittonic languages persisted.

In an interesting symmetry, many Northern Irish samples clustered strongly with southern Scottish and northern English samples, defining the Northern Irish/Cumbrian/Scottish (NICS) cluster group. More generally, by modelling Irish genomes as a linear mixture of haplotypes from British clusters, we found that Scottish and northern English samples donated more haplotypes to clusters in the north of Ireland than to the south, reflecting an overall correlation between Scottish/north English contribution and ChromoPainter PC1 position in Fig 1 (Linear regression: p < 2×10-16, r2 = 0.24).

North to south variation in Ireland and Britain are therefore not independent, reflecting major gene flow between the north of Ireland and Scotland (Fig 5) which resonates with three layers of historical contacts. First, the presence of individuals with strong Irish affinity among the third generation PoBI Scottish sample can be plausibly attributed to major economic migration from Ireland in the 19th and 20th centuries [6]. Second, the large proportion of Northern Irish who retain genomes indistinguishable from those sampled in Scotland accords with the major settlements (including the Ulster Plantation) of mainly Scottish farmers following the 16th Century Elizabethan conquest of Ireland which led to these forming the majority of the Ulster population. Third, the suspected Irish colonisation of Scotland through the Dál Riata maritime kingdom, which expanded across Ulster and the west coast of Scotland in the 6th and 7th centuries, linked to the introduction and spread of Gaelic languages [3]. Such a migratory event could work to homogenise older layers of Scottish population structure, in a similar manner as noted on the east coasts of Britain and Ireland. Earlier communications and movements across the Irish Sea are also likely, which at its narrowest point separates Ireland from Scotland by approximately 20 km.

ireland-britain-genetic-geography
Genes mirror geography in the British Isles. (A) fineSTRUCTURE clustering dendrogram for combined Irish and British data. Data principally split into Irish and British groups before subdividing into a total of 50 distinct clusters, which are combined into cluster groups for clusters that formed clades in the dendrogram, overlapped in principal component space (B) and were sampled from regions that are geographically contiguous. Names and labels follow the geographical provenance for the majority of data within the cluster group. Details for each cluster in the dendrogram are provided in S2 Fig. (B) Principal component analysis (PCA) of haplotypic similarity based on the ChromoPainter coancestry matrix, coloured by cluster group with their median locations labelled. We have chosen to present PC1 versus PC4 here as these components capture new information regarding correlation between haplotypic variation across Britain and Ireland and geography, while PC2 and PC3 (Fig 4) capture previously reported splitting for Orkney and Wales from Britain [7]. A map of Ireland and Britain is shown for comparison, coloured by sampling regions for cluster groups, the boundaries of which are defined by the Nomenclature of Territorial Units for Statistics (NUTS 2010), with some regions combined. Sampling regions are coloured by the cluster group with the majority presence in the sampling region; some sampling regions have significant minority cluster group representations as well, for example the Northern Ireland sampling region (UKN0; NUTS 2010) is majorly explained by the NICS cluster group but also has significant representation from the NLU cluster group. The PCA plot has been rotated clockwise by 5 degrees to highlight its similarity with the geographical map of the Ireland and Britain. NI, Northern Ireland; PC, principal component. Cluster groups that share names with groups from Fig 1 (NLU; SMN; CLN; CNN) have an average of 80% of their samples shared with the initial cluster groups. © EuroGeographics for the map and administrative boundaries, note some boundaries have been subsumed or modified to better reflect sampling regions.

Genomic footprints of migration into Ireland

Quite interesting is that it is haplogroups, and not admixture, that which defines the oldest migration layers into Ireland. Without evidence of paternal Y-DNA lineages we would probably not be able to ascertain the oldest migrations and languages broght by migrants, including Celtic languages:

Of all the European populations considered, ancestral influence in Irish genomes was best represented by modern Scandinavians and northern Europeans, with a significant single-date one-source admixture event overlapping the historical period of the Norse-Viking settlements in Ireland (p < 0.01; fit quality FQB > 0.985; Fig 6). (…) This suggests a contribution of historical Viking settlement to the contemporary Irish genome and contrasts with previous estimates of Viking ancestry in Ireland based on Y chromosome haplotypes, which have been very low [25]. The modern-day paucity of Norse-Viking Y chromosome haplotypes may be a consequence of drift with the small patrilineal effective population size, or could have social origins with Norse males having less influence after their military defeat and demise as an identifiable community in the 11th century, with persistence of the autosomal signal through recombination.

European admixture date estimates in northwest Ulster did not overlap the Viking age but did include the Norman period and the Plantations

The genetic legacies of the populations of Ireland and Britain are therefore extensively intertwined and, unlike admixture from northern Europe, too complex to model with GLOBETROTTER.

ireland-admixture-estimates
All-Ireland GLOBETROTTER admixture date estimates for European and British surrogate admixing populations. A summary of the date estimates and 95% confidence intervals for inferred admixture events into Ireland from European and British admixing sources is shown in (A), with ancestry proportion estimates for each historical source population for the two events and example coancestry curves shown in (B). In the coancestry curves Relative joint probability estimates the pairwise probability that two haplotype chunks separated by a given genetic distance come from the two modeled source populations respectively (ie FRA(8) and NOR-SG); if a single admixture event occurred, these curves are expected to decay exponentially at a rate corresponding to the number of generations since the event. The green fitted line describes this GLOBETROTTER fitted exponential decay for the coancestry curve. If the sources come from the same ancestral group the slope of this curve will be negative (as with FRA(8) vs FRA(8)), while a positive slope indicates that sources come from different admixing groups (as with FRA(8) vs NOR-SG). The adjacent bar plot shows the inferred genetic composition of the historical admixing sources modelled as a mixture of the sampled modern populations. A European admixture event was estimated by GLOBETROTTER corresponding to the historical record of the Viking age, with major contributions from sources similar to modern Scandinavians and northern Europeans and minor contributions from southern European-like sources. For admixture date estimates from British-like sources the influence of the Norman settlement and the Plantations could not be disentangled, with the point estimate date for admixture falling between these two eras and GLOBETROTTER unable to adequately resolve source and proportion details of admixture event (fit quality FQB< 0.985). The relative noise of the coancestry curves reflects the uncertainty of the British event. Cluster labels (for the European clustering dendrogram, see S4 Fig; for the PoBI clustering dendrogram, see S3 Fig): FRA(8), France cluster 8; NOR-SG, Norway, with significant minor representations from Sweden and Germany; SE_ENG, southeast England; N_SCOT(4) northern Scotland cluster 4.

Another study that strengthens the need to ascertain haplogroup-admixture differences between Yamna/Bell Beaker and Sredni Stog/Corded Ware.

Text and images from preprint article under a CC-BY-NC-ND 4.0 International license.

Featured image, from the article on Science Reports: The clustering of individuals with Irish and British ancestry based solely on genetics. Shown are 30 clusters identified by fineStructure from 2,103 Irish and British individuals. The dendrogram (left) shows the tree of clusters inferred by fineStructure and the map (right) shows the geographic origin of 192 Atlas Irish individuals and 1,611 British individuals from the Peoples of the British Isles (PoBI) cohort, labelled according to fineStructure cluster membership. Individuals are placed at the average latitude and longitude of either their great-grandparental (Atlas) or grandparental (PoBI) birthplaces. Great Britain is separated into England, Scotland, and Wales. The island of Ireland is split into the four Provinces; Ulster, Connacht, Leinster, and Munster. The outline of Britain was sourced from Global Administrative Areas (2012). GADM database of Global Administrative Areas, version 2.0. www.gadm.org. The outline of Ireland was sourced from Open Street Map Ireland, Copyright OpenStreetMap Contributors, (https://www.openstreetmap.ie/) – data available under the Open Database Licence. The figure was plotted in the statistical software language R46, version 3.4.1, with various packages.
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